WASHINGTON (AP) — Eager to cast blame, lawmakers are preparing to grill President Barack Obama’s top health official over problems with the rollout of the government’s health care website.

A growing number of Republicans in Congress are calling for Health and Human Services Secretary Kathleen Sebelius to step down or be fired because of problems consumers are having signing up for insurance coverage on the government’s new website.

On Wednesday, Sebelius heads to Capitol Hill to testify before the House Energy and Commerce Committee, her first appearance before Congress since state-based health exchanges opened for business on Oct. 1.

Sen. Lamar Alexander of Tennessee, the top Republican on the Senate Health Committee, on Tuesday joined the list of GOP lawmakers calling for Sebelius to go.

“Taxpayers have spent $ 400 million to create exchanges that, after 3½ years, still don’t work,” Alexander said. “No private-sector chief executive officer would escape accountability after such a poor performance.”

Sebelius is likely to face questions about problems with the website as well as a wave of cancellation notices hitting small businesses and individuals who buy their own insurance. Lawmakers also want to know how many people have enrolled in plans through the health exchanges, a number the Obama administration has so far refused to divulge.

On Tuesday, Medicare chief Marilyn Tavenner was questioned for nearly three hours by members of the House Ways and Means Committee who wanted to know why so many of their constituents were getting cancellation notices from their insurance companies.

“So what happened to the ‘If you like your insurance, you can keep it’ question?” asked Rep. Dave Camp, R-Mich., chairman of the Ways and Means Committee.

Camp was referring to one of Obama’s earliest promises about the health law: You can keep your plan if you like it. Obama’s promise dates back to June 2009, when Congress was starting to grapple with overhauling the health care system to cover uninsured Americans.

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Marilyn Tavenner, the administrator of the Centers for Medicare and Medicaid Services, testifies on …

As early as last spring, state insurance commissioners started giving insurers the option of canceling existing individual plans for 2014, because the coverage required under Obama’s law is significantly more robust. Some states directed insurers to issue cancellations. Large employer plans that cover most workers and their families are unlikely to be affected.

The law includes a complicated “grandfathering” system to try to make good on Obama’s pledge. It shields plans from the law’s requirements provided the plans themselves change very little. Insurers say it has proven impractical. The cancellation notices are now reaching policyholders.

Tavenner blamed insurance companies for cancelling the policies and said most people who lose coverage will be able to find better replacement plans in the health insurance exchanges, in some cases for less money. Change is a constant in the individual insurance market, she added, saying that about half of plans “churn” over in any given year.

Tavenner delivered the most direct mea culpa yet from the administration for the technical problems that have kept many Americans from signing up through HealthCare.gov. Consumers complain that the site is slow, locks up and often kicks them off before they can complete their application.

“I want to apologize to you that the website has not worked as well as it should,” she told the Ways and Means Committee.

Tavenner said the website is improving and the problems should be resolved by the end of November, giving consumers time to get coverage by the March 31 deadline.

The first senior official to publicly answer questions from lawmakers, Tavenner was also pressed about the number of people who have so far enrolled in health plans through the exchanges. Committee members asked Tavenner at least 19 times about the number of people who have so far enrolled through the exchanges.

Each time, she gave a well-rehearsed answer: “We will not have those numbers available until mid-November.”